In diesel engines, an exhaust gas recirculating system (EGR system) is widely used wherein a part of the exhaust gas is recirculated to lower the combustion temperature in order to reduce discharge of nitrogen oxide (NOx).
Tokkai Hei 6-249077 published by the Japanese Patent Office in 1994 discloses an EGR system fault detecting device wherein a differential pressure is detected before and after an EGR valve using a pressure sensor, and fault diagnosis is performed based on this differential pressure. Also, Tokkai Hei 7-42622 published by the Japanese Patent Office in 1995 discloses a device wherein a temperature sensor is provided in an EGR passage, and fault diagnosis is performed based on the temperature in the EGR passage.
Both these devices require a sensor to be provided to perform the diagnosis, and this increased the cost of diagnosis device.
There was also a problem in that it was not possible to diagnose if a fault occurs in the sensor.